Where the cladocerans came from

A group of scientists, including the researchers from the White Sea Biological Station, Lomonosov Moscow State University has studied dispersal routes of cladocerans through Northern Eurasia, which are a food for many fish species. The scientists have shown that at least several cladoceran taxa began collonization of the whole Palearctic right from its north, and a part of them – from the Beringian region. Keeping in mind that the Bering Strait closed numerous times in the past, current Kamchatka, Chukotka, Alaska and a part of Aleutian Islands were a part of so called Beringia – a large part of land with much larger area than nowadays. The results of this study are published in PLOS ONE journal.

The technical part of the project – namely determination of the nucleotide sequence in some genes of the cladocerans – was fulfilled at the White Sea Biological Station, Lomonosov Moscow State University. Tatyana Neretina, Ph.D., a researcher of the Biological station and research co-author notes: "It could seem ridiculous to bring samples to the White Sea, to Polar Circle, but our laboratory is equipped so well and it's so convenient to work there that colleagues come to us and work with our collections, as well as with samples from all over the world, including that ones, gathered in Antarctica by the "Polarstern" research icebreaker. In most cases we study samples in order to understand, what species inhabit these high latitudes. However, in this particular case we've studied biogeography."

Egg-travellers

Freshwater invertebrates, and among them small crustaceans, turn out to be a cornerstone of many ecosystems, and evolution of their lines is closely intertwined with fates of many other species. All these have created the basis of biodiversity development in the Northern Hemisphere. Cladocerans' eggs could travel from one water body to another whitin special containers left after molting, which are called ephippiua, and disperse to new inhabiting areas (for instance, on feathers of some water birds).

The Russian team of hydrobiologists from Nikolai Smirnov's scientific school (who is one of the greatest specialists in carcinology in the XXth century) has devoted its research to this group of organisms. The published paper is dedicated to 89th aniversary of him.

Biogeography deals with investigations of dispersal routes of living organisms. According to the opinion of this paper authors, biogeography of freshwater animals is much less developed, but and dispersion patterns in freshwater animals strongly differ from thouse of terrestrial inhabitants. It happens due to differences in the biology and geological age of freshwater and terrestrial animals.

Where have "horned" small crustaceans come from?

It's impossible to study all groups of organisms in one research, that's why scientists usually choose particularly specific groups as their models. Phylogeography – a science studying a distribution of genetic lines and dealing with the analysis of the dispersion peculiarities of such groups.

Previous researchers studied almost only cladocerans belonging to the genus Daphnia. According to these data, Japan was the center of dispersion of several groups of species across Eastern Eurasia. In order to check such conclusions on other groups of the cladocerans, hydrobiologists are conducting now their genetic analysis. A strong progress in this direction has been achieved by Russian scientists by means of a study of the cladocerans from the Chydorus sphaericus group, which are common and most typical inhabitants of the northern regions. To achieve this aim, scientists have investigated the samples of cladocerans from different regions of Russia, including Kronotsky and Komandorsky Nature Reserves, as well as samples from Norway and South Korea.

In order to reveal the centers of dispersion, scientists compare features of descendants and ancestors of a group. Afterwards, they look for taxa, which have more ancestral features as these taxa are regarded as nearest to an initial group. Such comparisons have been made previously based exclusively on morphological features, however, modern scientists understand that it's not enough for adequate conclusions. Tatyana Neretina explains: "Studying only morphological features could say a little about how they evolve and disperse. That's why nowadays every zoologist and botanist needs to know molecular genetic methods."

Routes of cladocerans and zoologists

The molecular genetic laboratory was established in 2006 at the White Sea Biological Station due to the activity of Tatyana Neretina and Nikolai Mugue, Ph.Ds, and the first international workshop on molecular methods in zoology was also conducted there in 2009. Eugenia Bekker, who was at that time a Ph.D. student under the supervision of Alexey Kotov, a project co-author from A.N. Severtsov Institute of Ecology and Evolution, was among the participants of this workshop.

Comparison of the gene sequences allows to understand, how old is the division of different lines and groups of living organisms. The speed of mutation accumulation varies among different genes. Most important genes are less variable, and vice versa. As a result, scientists apply "molecular clocks" for determining of congeniality and age of divergence of various groups of living organisms.

Tatyana Neretina continues: "You collect material from different places, preserve it in alcohol, extract DNA and after that with the help of polymerase chain reaction (PCR) determine the order of nucleotides in the genome fragment, you need, in different samples and look at the similarity between different specimens. This allows to form phylogenetic trees, which help to understand the relationships between organisms."

In case of the Chydorus sphaericus group we've studied two genes: 1) a portion of mitochondrial gene (containing in a mitochondria – power producers in each cell) encoding an important enzyme – cytochrome c; 2) a portion of the nuclear DNA, where two parts of ribosomal RNA (necessary for protein synthesis in a cell) are coded and the indel between them.

Small crustaceans have survived glaciation in the north.

Comparison of gene sequences have helped hydrobiologists to determine relationships between groups and trace pattern of their dispersion across Northern Eurasia. For this purpose scientist have built median-joining haplotype networks (a haplotype is a group of organisms, which all have exactly the same gene sequence) in order to determine regions, where ancestral groups of Chydorus sphaericus inhabit, and understand, from which region they colonized other territories and where their initial center of dispersion is located. It was found that centers of dispersion of two groups were located in the European part of the continent, when of two others – in the Asian one. One clade survived during harsh environment of the Pleistocene glaciation in a northern refugium – at the territory of current Arkhangelsk Oblast and the Komi Autonomous Republic, while another clade did it in the south (what was aleady shown numerous times for other species of animals in the previous studies).

Tatyana Neretina comments: "The concept of a refugium implies the absence of ice there, and it's not so important whether it was southern or northern. No special adaptations are needed in order to survive glaciation inside a refugium. In contrast, our study shows that in refugia the animals already had all necessary adaptations to be widely dispersed".

Another group of Chydorus sphaericus colonized Bering Island from the continental Beringian region (the former was not connected with the latter in the past) but three times independently, not simultaneously. Another result of this project lies in a preliminary detection of the area of secondary contact between the Beringian and European Siberian super-complexes in the Yenisey River basin. Moreover, similar conclusion has been made by the same research group for the other cladoceran genus – Moina. Finally, Eastern Siberia and Northern Atlantic (Greenland and Iceland) region are contain some relict endemic clades.

In the future this scientific team, which also includes researchers from the Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences, Institute for Inlanf Water Biology of Russian Academy of Sciences and the Faculty of Biology of the State University of New York, is going to test applicability of revealed scheme flexibility to other cladoceran genera.